The present invention relates to a countershaft-type transmission having a plurality of parallel intermeshing gear trains on and between parallel shafts, the gear trains being selectable for different gear positions, and more particularly to such a transmission with one-way clutches associated respectively with two high-load gear trains. The present invention is also directed to a method of and an apparatus for controlling the operation of a transmission through the control of operation of clutches, and more particularly to a method of and an apparatus for controlling a transmission which has one-way clutches in a power transmitting path thereof and an engine-brake clutch for locking the one-way clutches.
Countershaft-type transmissions have parallel shafts supporting intermeshing gears which provide a plurality of parallel gear trains for the transmission of engine power. When one of the gear trains is selected for engine power transmission, the transmission sets up a corresponding gear position. Such countershaft-type transmissions are widely used as automotive transmissions. One example of countershaft-type transmission is disclosed in Japanese Laid-Open Patent Publication No. 61(1986)-233248, for example.
Some countershaft-type transmissions include one-way clutches combined with gear trains (especially, high-load gear trains), for allowing the engine power to be transmitted only in a direction to drive the automobile. Japanese Patent Application No. 63(1988)-180647, for example, shows a countershaft-type transmission wherein one-way clutches are associated respectively with two high-load gear trains.
However, use of such one-way clutches prevents engine brake from being applied when the engine power is to be transmitted in the direction opposite to the automobile driving direction because the one-way clutches would idly rotate and fail to transmit the engine power. One solution is to include an engine-brake clutch which directly couples the input and output members of a one-way clutch thereby to lock the one-way clutch, so that engine brake is available.
Transmissions with engine-brake clutches tend to be long in the axial direction since they require a space for the installation of the engine-brake clutch in combination with the one-way clutches. In the transmission disclosed in Japanese Patent Application No. 63(1988)-180647, particularly, two one-way clutches are disposed in series with each other, and their operation is controlled by a single engine-brake clutch. It is necessary to couple the input member of one of the one-way clutches to the output member of the other one-way clutch. The gear trains, clutches for selecting the gear trains, and members by which the input and output members of the one-way clutches are arranged such that the transmission is necessarily large in size.
The layout of the above transmission components should preferably be designed to position the high-load gear trains at an end of the axial span or length of the transmission thereby minimizing any bending moment on the shafts and any flexing of the shafts. Moreover, the two high-load gear trains combined with the one-way clutches should be located as close to each other as possible thus allowing the one-way clutches to be easily coupled to each other.
Countershaft-type transmissions are often used as automatic transmissions. In countershaft-type automatic transmissions, one-way clutches are also combined with gear trains for allowing the engine power to be transmitted only in a direction to drive the automobile. Since such a one-way clutch prevents engine brake from being applied, an engine-brake clutch is also disposed parallel to the one-way clutch. To apply engine brake, the engine-brake clutch is engaged to lock the one-way clutch.
Automatic transmissions on automobiles are operated to automatically select gear positions depending on the running conditions of the automobiles. One automatic transmission is disclosed in Japanese Patent Publication No. 52(1977)-21131. An example of the process of controlling an automatic transmission is shown in Japanese Laid-Open Patent Publication No. 61(1986)-189354.
Generally, an automatic transmission comprises a fluid-operated power transmission mechanism such as a torque converter and a plurality of power transmitting paths such as gear trains which are coupled to the output shaft of the power transmission mechanism. The transmission can be shifted to any of various gear positions when a corresponding one of the power transmitting paths is selected. According to one generally known arrangement, selection of the power transmitting paths is carried out by shift control valves which control the operation of hydraulically-operated clutches (shift means) in the respective power transmitting paths.
As is well known in the art, the shift control valves and a control valve for controlling an engine-brake clutch are controlled in operation by solenoid valves because complex operation of these control valves can accurately be controlled through electric control of the solenoid valves. However, if the solenoid valves fail, then the control of the control valves will become inaccurate. Failures of the solenoid valves include breakage of wires and short circuits. It is empirically found out that solenoid valve short circuits are less probable, and measures against wire breakage should preferably be taken in advance.
The automatic transmission operates selectively in shift control modes in which engine brake is needed and shift control modes in which no engine brake is needed. Under certain conditions, the engine-brake clutch is engaged to apply engine brake, and under other conditions, the engine-brake clutch is disengaged.
Heretofore, those conditions for engaging and disengaging the engine-brake clutch are uniquely set up for corresponding shift ranges (e.g., D, 2, 1 ranges) which are selected by manual valves operated by the shift lever of the transmission. More specifically, for the D range, a power transmitting path with a one-way clutch is set up with the engine-brake clutch disengaged to apply no engine brake. For the 2 and 1 ranges, a power transmitting path is set up with the engine-brake clutch engaged to apply engine brake.
When the shift lever is operated to shift the transmission from the D range to the 2 or 1 range, for example, making a downshift while the accelerator pedal is being depressed (power-on mode), the input and output members of the shift clutch are synchronized, and then the engine power starts being transmitted through the one-way clutch. Therefore, the transmission can smoothly be shifted. However, as described above, the engine-brake clutch is engaged or disengaged depending on the shift range selected by the shift lever. The transmission is shifted from the D range to the 2 or 1 range while the engine-brake clutch is being engaged. Consequently, the engine-brake clutch is engaged before the input and output members of the shift clutch are synchronized, resulting in a shift shock which leads to jerky shifting operation.
One conventional solution has been to employ an accumulator disposed in an oil passage which supplies working oil to the engine-brake clutch, so that the engine-brake clutch will not abruptly be engaged, thereby reducing any shift shock. With this arrangement, however, a control valve requires the incorporation of an accumulator, and is complex and large in size.